There are a variety of imaging techniques that have been used to diagnose disease in humans. One of the first techniques employed was X-rays. In X-rays, the images of the patients' body reflect the different densities of body structures. To improve the diagnostic utility of this imaging technique, contrast agents are employed to increase the density difference between various structures, such as between the gastrointestinal tract and its surrounding tissues. Barium sulfate and iodinated contrast media are the two primary pharmaceutical oral contrast agents used for X-ray gastrointestinal studies to visualize the esophagus, stomach, small intestine, and large intestine. Likewise, these same contrast agents are used for X-ray-based CT images to improve visualization and distension of the gastrointestinal tract and to provide improved contrast between the gastrointestinal tract and the structures adjacent to it, such as the blood vessels and lymph nodes. Such gastrointestinal oral contrast agents increase the density inside the esophagus, stomach, small intestine, and large intestine, and allow differentiation of the gastrointestinal system from the surrounding structures. Such gastrointestinal oral contrast agents also distend the bowel lumen allowing better evaluation of bowel wall structures, including abnormalities such as wall thickening or luminal obstruction.
Magnetic resonance imaging is another imaging technique; however, unlike X-rays and CT, MR does not utilize ionizing radiation. MR employs a magnetic field, radiofrequency energy and magnetic field gradients to make images of the body. The contrast or signal intensity differences between tissues mainly reflect the T1 and T2 relaxation values and the proton densities of the tissues. Like CT, MR can make cross-sectional images of the body and it is desirable for many MR scans that a gastrointestinal contrast agent be administered to allow differentiation of the gastrointestinal system from the surrounding structures.
Opacification of the gastrointestinal tract is a routine part of patient preparation prior to CT examination of the abdomen and pelvis, because proper filling and distention of the bowel lumen with contrast medium facilitates identification of the normal and abnormal anatomy. For this reason, oral contrast agents are administered to the vast majority of patients undergoing CT scans of the abdomen and pelvis. Less commonly, oral contrast agents are administered to patients undergoing MR scans of the abdomen and pelvis.
The properties of an ideal oral contrast agent for CT or MR include the following: good taste and pleasing consistency; low cost; premixed with a long shelf life; predictable and dependable transit time; absence of side effects or allergic reaction; non-absorbable; chemically inert, lack of precipitation or loss of suspension; low incidence of streak or magnet-susceptibility artifacts; Hounsfield unit (HU) opacity (contrast density or degree of brightness) for CT, and either hyperintensity (“bright”) or hypointensity (“dark”) for MR, which allow easy differentiation of bowel loops from adjacent structures and also allow easy visualization of the contrast-enhanced bowel wall. Unfortunately, no single agent fulfills all of these criteria. For example, sometimes a clinician desires a fast transit time of the oral contrast agent through the bowel whereas at other times a slow transit time is desired. Sometimes a clinician desires a bright contrast agent (which appears as white on CT or MR), often termed a positive contrast agent. At other times the clinician may desire a neutral or dark contrast agent (which appears gray or black, respectively, on CT or MR), often termed a negative contrast agent.
The Hounsfield scale is a quantitative scale for describing radiodensity. The Hounsfield unit (HU) scale is a linear transformation of the original linear attenuation coefficient measurement in which the radiodensity of distilled water at standard pressure and temperature is defined as zero HU, while the radiodensity of air is defined as −1000 HU. For comparison, the HU of various body structures include fat at −120 HU, muscle at +40 HU, and compact bone at +1000 HU.
In the early 1980s, when CT scan imaging was first gaining widespread use, the first oral agents used were the ionic iodinated contrast agents, comprised of the iodine-containing salts sodium diatrizoate, meglumine diatrizoate, or mixtures thereof. One example of the ionic iodinated oral contrast agents is Gastrografin® (Bracco Diagnostics, Princeton, N.J.), which is composed of the iodine-containing salts meglumine diatrizoate and sodium diatrizoate. Pharmaceutical agents like Gastrografin® were already approved for oral use and were widely used in full strength concentration for diagnostic gastrointestinal radiology imaging. These products, as well as the newer non-ionic iodinated agent approved for oral administration in the United States (e.g., iohexol, trade name Omnipaque®, GE Healthcare, Princeton, N.J.), were too concentrated in the form supplied by the manufacturers to be used for gastrointestinal opacification during CT imaging. These products were, therefore, diluted with water or other beverage in order to be of the proper concentration for optimal CT scan imaging.
Similarly, the gadolinium-based oral contrast agents, such as gadopentetate dimeglumine, are too concentrated in the form supplied by the manufacturers to be used for gastrointestinal opacification during MR imaging. These products must be diluted with water or other beverage in order to be of the proper concentration for optimal gastrointestinal opacification during MR imaging.
In addition to the use of iodine-based agents, barium sulfate-based oral contrast agents for CT scan imaging were developed and perfected, and these products are used by many radiology imaging centers. There are currently various formulations of the barium sulfate oral contrast agents, each with its unique imaging characteristics. Formulations vary in terms of the degree of Hounsfield unit opacification (higher HU correlates with a greater degree of brightness, or whiteness, on the CT image), transit time through the bowel, viscosity, ability to distend bowel loops, and taste/palatability.
Iodinated contrast agents intended for oral administration prior to CT imaging are usually mixed with either water or a commercial, artificially-sweetened beverage such as Kool-Aid® (Kraft Foods North America, Rye Brook, N.Y.). The clinician is able to vary the initial HU opacification (contrast density) by altering the ratio of oral contrast agent to beverage diluent. The clinician is also able to vary the palatability by choosing different beverages for dilution.
The ionic (e.g., sodium diatrizoate and meglumine diatrizoate) iodinated contrast agents for CT, the non-ionic (e.g., iohexol) iodinated contrast agents for CT, and the gadolinium-based (e.g., gadopentetate dimeglumine) contrast agents for MR have inherent differences in their chemical and physical properties that result in clinical differences in their application to gastrointestinal tract imaging, as well as significant differences in their palatability.
There is, therefore, a need for a ready-mixed range of liquid dilution vehicles that allows the clinician to vary the osmolality of the administered oral contrast (which affects transit time through the bowel, oral contrast density, and bowel distension), to vary the viscosity of the administered oral contrast (which affects the transit time and the degree of bowel lumen distension), to vary the pH and buffering capacity of the administered oral contrast (which affects the chance of precipitation or denaturation of the oral contrast when exposed to the low pH environment of the stomach), and to vary the flavor/sweetness of the liquid vehicle to best mask the inherent palatability issues unique to the oral contrast agent utilized.
This range of liquid dilution vehicles allows the clinician to match the appropriate vehicle characteristics to the inherent differences in the imaging and chemical characteristics of the various oral contrast agents. Furthermore, the clinician can vary the liquid vehicle/oral contrast agent mixture and concentration to match the desired imaging characteristics for a particular patient's imaging scan.
The liquid vehicle of the invention is designed to be used only with water-soluble oral contrast agents such as the iodine-based and gadolinium-based agents. Non water-soluble oral contrast agents such as barium sulfate and bentonite are not properly suspended by the liquid vehicle and are not suitable for admixture with the liquid vehicle of the invention.